This invention relates generally to optical channel waveguides and, more particularly, for techniques for obtaining a desired optical reflection and transmission filter response in optical channel waveguides. Optical waveguides have a variety of both military and commercial applications, such as in optical signal processing, optical networking and related communication systems. In processing optical signals, there is frequently a need to provide filtering function and one well known way of achieving this is an optical grating, such as a Bragg grating, integrated into a waveguide. A grating is a periodic structure either formed in the surface of a waveguide or embedded in the volume of the waveguide as variations in refractive index. Regardless of its form, the grating provides periodic perturbation of a propagation constant associated with optical energy propagating through the waveguide. Reflections from the grating elements may interfere constructively or destructively, affecting the reflection and transmission filter response characteristic of the waveguide.
Optical waveguides, of course, take various forms, such as optical fibers, planar waveguides and other waveguides of various cross sectional shapes. The present invention is concerned, however, with optical channel waveguides, which have some practical advantages over other types of waveguides. In particular, optical channel waveguides may be conveniently fabricated using conventional lithographic techniques and may, therefore, be easily integrated with electro-optical and electronic components. An optical channel waveguide can be formed by modifying a property of a suitable substrate to provide confinement of optical energy to a narrow channel at or near the surface of the substrate. For example, a channel waveguide may be formed on a substrate of dielectric material, such as lithium niobate (LiNbO3), often referred to by the abbreviation LNO. For example, a known channel waveguide structure includes a channel region formed by the diffusion of a metal, such as titanium (Ti) into a planar surface of the LNO material. The Ti:LNO channel region has a slightly higher refractive index than the surrounding LNO material and functions as a channel waveguide when light is launched into it.
An optical grating may be formed on or in an optical channel waveguide, either in the form of surface relief grating elements extending perpendicular to the direction of light propagation along the channel waveguide, or by modulating the refractive index within the channel structure. Although the principles of optical channel waveguides are well known, and the use of optical gratings in conjunction with optical channel waveguides is known, no-one prior to the present invention has conceived a technique for selectively controlling the characteristics of optical gratings in a convenient manner to achieve desired optical filter characteristics.
It is known that the physical structure of an optical grating may be apodized, i.e., selectively weighted, in some manner to achieve a desired filter characteristics. A paper by D. Wiesmann et al., “Apodized Surface-Corrugated Gratings With Varying Duty Cycles,” IEEE Photonics Technology Letters, Vol. 12, No. 6, June 2000, pp. 639-641, proposes that surface corrugations forming a Bragg grating may be apodized by varying the duty cycle of the corrugations. That is to say, the length of the corrugations as measured in the principal direction of propagation (perpendicular to the direction in which the parallel corrugations are oriented) is varied while the periodic spacing between corrugations is maintained constant. Varying the grating duty cycle requires varying the size of each grating ridge, from the smallest achievable size to the largest achievable while still forming a grating groove between ridges. This requires a very precise, and therefore costly, fabrication technique, which is probably why the authors of the paper referenced above described the proposed structure as best implemented using electron beam lithography.
It will be appreciated from the foregoing that there is a need for a more convenient, reliable and economical technique for forming an apodized optical grating on or in an optical channel waveguide. The present invention is directed to this end.